Silver halide photographic materials are light sensitive. Prior to exposure and development they must be protected agaist unintended exposure to light. Carbon black is a common opacifying agent that is incorporated in or coated on a support and associated with photographic materials containing silver halide in order to protect them from unwanted exposure. The opaque support can be non-light sensitive and associated with the silver halide element as a packaged material for sheets or rolls of photographic material, or as a backing material.
A backing material is an opaque sheet placed on the side of a photographic element opposite the side bearing the silver halide emulsion layer. Unlike a packaging material, the backing material remains associated with the photographic element while it is being exposed. Its principal function is to carry indicia regarding the photographic element which are viewable through the back of a camera, and at the same time protect the photographic element from exposure to light entering through light transmissive portions of the back of the camera.
A problem has been observed recently with uniform fog occurring in photographic elements which have a backing material associated with them. This has been observed in particular with color photographic elements containing silver halide emulsion layers which have been sensitized with sulfur and gold.
The present invention is based on the discovery of the origin of and means for reducing this objectionable fog.
We have found that the fog is caused by hydrogen cyanide gas which evolves from the carbon in an associated backing material. We believe the mechanism which occurs is that hydrogen cyanide gas evolves from the carbon and migrates to the silver halide emulsion where it binds with the gold in sulfur and gold sensitized silver halide grains. This leaves silver sulfide fog centers which lead to development of unexposed silver halide grains upon processing.
As a consequence of these discoveries, we have recognized that the objectionable fog can be eliminatd by incorporation of a scavenger for hydrogen cyanide gas in the backing material. It will be recognized that while the description hereinafter refers to backing materials, carbon contained in packaging materials, or in other non-light sensitive materials associated with a silver halide photographic element, could also cause a fog problem, and that the fog would be eliminated by applying to these other materials the same solution as is applied to backing materials.
It has been suggested in U.S. Pat. No. 3,900,323 that certain heavy metal salts can be added to backing or antihalation layers of photosensitive elements containing carbon black in order to inhibit fog formation attributed to sulfur or sulfur containing compounds associated with the carbon black. This patent does not suggest the need to scavenge hydrogen cyanide gas or the effectiveness of the scavengers of this invention in the amounts in which they are employed.
In accordance with the present invention there is provided a photographic assemblage comprising
a light-sensitive photographic element comprising a support bearing a silver halide emulsion layer and
an associated non-light sensitive material comprising a support containing carbon black,
the assemblage containing, as a scavenger for hydrogen cyanide gas in a location where it can intercept hydrogen cyanide gas evolving from carbon black in the associated material before the gas reaches the silver halide in the light-sensitive element, a noble metal compound selected from the group consisting of palladium, gold, platinum, in an amount of about 1 to 250 parts metal per million parts carbon black by weight or a noble metal compound selected from the group consisting of iridium, rhodium and osmium in an amount of about 10 to 1,000 parts metal per million parts carbon black by weight.
It is believed that the heavy metal scavenger compound binds with hydrogen cyanide gas to form a photographically inert material. The scavenger either should not itself have a deleterious effect on silver halide or should be incorporated in such a manner that it does not come into contact with the silver halide layers. It should not release a material as a result of scavenging cyanide gas which would have a deleterious effect on the silver halide material. Suitable scavengers for cyanide gas can be selected from inorganic or organic compounds of noble metals. Especially preferred are compounds of palladium (II or IV), platinum (II or IV), and gold (I or III). Compounds of rhodium (III), iridium (III or IV), and osmium (II, III or IV) are also effective, but are less preferred because of the higher amount needed to obtain equivalent protection.
Typical useful compounds include potassium tetrachloropalladate (II), potassium hexachloropalladate (IV), palladium (II) chloride, palladium (II) acetate, potassium tetrachloroplatinate (II), rhodium (III) chloride, potassium tetrachloroaurate (III), potassium hexachloroplatinte (IV), palladium (II) tetraamine dichloride, potassium hexachloroiridate (IV), potassium tetrachloroiridate (III), and potassium hexachloroosmate (IV).
The scavenger materials are commercially available and should be used in a degree of purity which would not deleteriously affect the photographic material. Depending upon the location of the scavenger material, differing degrees of purity can be used.
Trivelli and Smith U.S. Pat. Nos. 2,566,245 and 2,566,263 describe the use of certain compounds of palladium, platinum, iridium, and rhodium as fog-inhibiting compounds for silver halide emulsions to improve keeping under high humidity and high temperature conditions, as in tropical regions. These patents describe the use of certain noble metal salts as addenda to the silver halide emulsion before coating, either before digestion of the silver halide, before spectral sensitization, or after spectral sensitization. They also describe their use in a layer contiguous or adjacent to, or in contact with the silver halide emulsion layer. There is no suggestion in these patents that the fog is caused by cyanide, nor that the noble metal salts specified might protect from cyanide from sources outside the emulsion, nor that the salts might be effective if not contained in the emulsion layer or an adjacent layer.
In the present invention, the scavenger material preferred is located in a position close to the source of the cyanide gas, although it can be located at a more remote location, so long as it can intercept cyanide gas before it reaches the silver halide emulsion. Thus, it can be located in the layer that contains the carbon black, it can be located in a layer coated over the layer containing the carbon black, it can be coated in a layer on the opposite side of the support from the carbon black layer or it can be located in a layer of the photographic element remote from the silver halide emulsion layer.
The amount ofpscavenger will depend upon the efficacy of the particular scavenger employed, the location in which it is placed, the type of carbon which is used, the amount of cyanide which is expected to evolve from the carbon and the proportion of evolved cyanide which is desirably intercepted. Expressed in terms of parts metal per million parts carbon black, the more effective scavengers, such as platinum, palladium and gold, are employed in an amount of about 1 to 250 parts metal per million parts carbon by weight. Preferred are amounts of about 5 to 70 ppm metal and especially preferred are 5 to 25 ppm metal. The less effective scavengers, such as iridium, rhodium and osmium are employed in amounts of about 10 to 1000 parts metal per million parts carbon, by weight. Especially preferred are amounts of about 100 to 1000 ppm metal.
The carbon with which the present invention can be employed is any carbon black which can be used as an opacifying material in photographic backing and packaging materials. We have determined that the amount of cyanide evolved from a carbon black varies depending upon the conditions under which the carbon black was prepared. Thus, the more cyanide gas which can evolve from a carbon black the greater the need for our invention. Examples of such carbon blacks are furnace blacks, lamp blacks, and channel blacks. Among those with which our invention can be used, are carbon blacks sold under such trademarks as Neotex and Raven by Columbian Chemicals Company, Swartz, La. 71281 and under such trademarks as Monarch, Regal and Black Pearl by Cabot Corporation, Boston, Mass. 02110.
The backing or packaging material can be a simple element comprising a support in which is incorporated the carbon black, or which bears one or more layers of carbon black. It can, however, contain additional layers such as overcoat layers and a layer or layers which contain printed information. One or more carbon black layers can be employed.
The support is a planar material, typically a paper support or a "synthetic" paper support composed of a polymer intended to simulate paper. However, the support can be composed of a film of polyester or polyolefin. The support should have the requisite flexibility to serve its intended function as a backing or packaging material.
The carbon black layer will comprise carbon black dispersed in a binder. Useful binders include naturally occurring polymeric vehicles such as gelatin and cellulose derivatives and synthetic vehicles such as polyvinyl alcohol and its derivatives, acrylate and methacrylate polymers, butadiene-styrene polymers and similar materials. The various layers of the element can contain other components such as surfactants, dispersing aids, coating aids, other pigments, and the like.
When the scavenger for cyanide gas is incorporated in the carbon black layer it conveniently will be dispersed in the same binder as is employed for the carbon black. If it is employed in another layer, such as an overcoat layer, it can be dispersed in any useful binder, such as those enumerated above, which would be compatible with the adjacent layers. Alternatively, it can be applied without a binder from an aqueous or organic solvent.
The photographic element with which the backing or packaging material is associated can be any photographic element known in the art. It can be a simple element comprising a support bearing a layer of a silver halide emulsion. In normal practice it will be more complex. The photographic element can be a black and white element intended for amateur or professional use, including radiographic use, or it can be a color photographic material intended to form a color negative image or a color positive image.
As indicated above, the present invention is particularly useful in reducing fog formation in silver halide emulsions which are sensitized with gold, such as sulfur and gold, selenium and gold, etc. Such sensitization is described in more detail in U.S. Pat. Nos. 2,743,182 and 3,297,447, the disclosures of which are incorporated herein by reference.
The photographic materials will commonly be spectrally sensitized with a spectral sensitizing dye. The color photographic elements will have multiple layers which are sensitive to different regions of the visible spectrum and commonly will have associated with each a color former, such as a dye-forming coupler, that will provide a viewable dye image.
Further details of such materials can be found in Research Disclosure, December 1978, item 17643, which relates to photographic silver halide emulsions, preparations, addenda, processing and systems; Research Disclosure, November 1979, item 18716, which relates to reflection color print materials; and Research Disclosure, August 1979, items 18431, which relates to radiographic materials. Research Disclosure is published by Industrial Opportunities Ltd., Homewell, Havant, Hampshire P09 1EF UK.
In the following examples, which further illustrate this invention, the following Standard Test Procedure was employed:
Backing and packaging materials were evaluated by the amount of fog induced in a color negative film containing sulfur and gold sensitized silver halide emulsions when the film was left in contact with the material. The film employed contains a sulfur and gold sensitized silver bromoiodide emulsion and is sold under the trademark Kodacolor VRG 100. When exposed to hydrogen cyanide, fog is exhibited first in one of the green-sensitive layers. It appears as an increase in green minimum density (D.sub.min). As the amount of hydrogen cyanide increases, fog is exhibited in other layers. The standard test procedures used is as follows:
A paper sample was cut into two strips 3.5 cm by 30 cm. These were maintained for one hour at 50% relative humidity and 25.degree. C. The strips were placed one on each side of a 3.5 cm by 30 cm strip of the film sample, and sealed in an aluminum foil bag lined with polyethylene. The sealed bag was maintained at 49.degree. C. and 50% relative humidity for seven days, after which the film was processed (without exposure) in the Kodacolor C41 process, the details of which are described in the British Journal of Photography Annual, 1977, pp. 201-205. The amount of fog was determined from the difference in green density between the film incubated with the test paper and that of a piece of film incubated under the same conditions without the paper.